Lithotripters are nowadays widespread as medical devices for destroying concrements, for example kidney stones, in the body of a patient with the help of focused shockwaves. Such lithotripters are commercially offered by the Applicant, for example under the designation “Dornier Lithotripter S” or “Dornier Compact Delta®.” In all of these devices, the concrement must be located prior to the beginning of the treatment so that the patient can be positioned with the help of a movable stretcher such that, for example, his kidney stone is within the focus of the shockwaves that are generated with the help of the shockwave generator of the lithotripter. This initial “adjustment,” i.e. positioning of the patient, is normally carried out with an imaging locating device, for example an imaging ultrasonic scanner or an X-ray locating device. In addition to the initial locating of the concrement for the first time before the beginning of the ESWL (extracorporeal shockwave lithotripsy) treatment or ESWT (extracorporeal shockwave therapy) treatment, this device also serves to continuously monitor the position of the concrement during the treatment in order to make sure that it has not slipped within the patient's body or traveled to another place or that the patient has not moved on his stretcher, so that the concrement might no longer be in the focus of the shockwaves. For a comprehensive survey of technical and medical aspects of ESWT and of the devices used in lithotripters, reference can be made to the book “ESWT and Ultrasound Imaging of the Musculoskeletal System,” Steinkopff-Verlag, Darmstadt, 2001, ISBN 3-7985-1252-3.
As a general rule, the imaging locating device is also used to monitor the success of lithotripsy in the course of the treatment, i.e. fragmentation of the target object. Since a treatment typically takes about 30 minutes, it is usually not feasible to complete the treatment in a continuous manner, because of the excessive radiation dosage of X-rays. Therefore, a treatment is typically conducted during intervals of 3 to 5 minutes. If the target object shifts during the treatment intervals, for example due to a movement of the patient, the patient's body will be loaded by shockwaves, until the next control inspection, without the target object being further fragmented, since it is no longer within the shockwave focus.
In lithotripters in which an ultrasonic scanner is used as the imaging locating device, the scanner may be used continuously for visualizing the target object, but the locating operation is often much more difficult than in radiograms, among other things, because of the image quality, so that even experienced medical personnel will often have difficulties in recognizing the target object or even in evaluating the degree of fragmentation of the object. Moreover, the use of such imaging locating devices not only for the “initial adjustment” of the patient, but also for “hit control” in the course of the treatment has a fundamental drawback. That is, although typically the position of the target object relative to the shockwave focus can thereby be controlled, for example by a purely geometrical quantity, the effect of the shockwaves on the target object itself cannot be detected. Fundamental problems such as an inadequate coupling of the shockwave device to the patient's body, a vignetting of shockwaves (e.g., by ribs), etc., might therefore be detected very late, such as when no effects become visible on the target object in the course of the treatment.
To address the foregoing problems, special ultrasonic methods, particularly ultrasonic Doppler methods, for continuous hit control have been suggested in existing methods, based on the assumption that, when hit, the concrement in the human body will perform a macroscopic movement due to pulse transmission from the shockwave. When the target object is exposed to ultrasonic waves and when the ultrasonic waves reflected on the object are measured, this macroscopic movement will be expressed in a Doppler shift of the frequency of the reflected waves. Lithotripters equipped in this way, where the evaluation of the received ultrasonic waves includes a Doppler analysis, are known, for example, from EP 0 367 116 B1, EP 0 548 048 B1, and DE 44 46 192 A1. These devices have in common that the ultrasonic transducer emits ultrasonic waves into the body and that the ultrasonic waves reflected by the body back to the ultrasonic transducer are sensed, with a Doppler signal unit, which consists of the emitted and received ultrasonic waves, generating and evaluating a Doppler signal, wherein essentially the amount of a frequency shift of the reflected signal relative to the emitted waves is calculated and the hit accuracy is inferred therefrom.
However, the foregoing approach has various drawbacks. For example, it is known that the Doppler signal includes artifacts, especially near its zero point in time (i.e., directly after emission of the shockwave). To prevent the measurement of such artifacts, the lithotripters according to EP 0 367 116 B1 and EP 0 548 048 B1 are, for example, equipped with means for the time synchronization between the shockwave generator and the Doppler signal unit, which makes these devices complicated and expensive. Moreover, in the vicinity of strong scatterers, e.g. a concrement, there may be the so-called mirror artifact in the Doppler spectrum. Due to double reflection, the movement of a scatterer is additionally recorded in the opposite direction. This leads to an additional amount, which corresponds to the useful signal reflected at the zero line. Due to the briefness of the process and the strong artifacts, velocity-time curves, and thus hit information, cannot easily be assigned to the spectra.
Therefore, there is a need in the art to provide a system and method for a lithotripter that determines and displays information about hit control and disintegration control on the basis of received ultrasonic waves, without the need for performing a Doppler analysis, such as that described above, which requires complicated apparatus and is prone to errors.